TEMPERATURE COEFFICIENTS OF THE RATES OF CL ATOM REACTIONS WITH C2H6, C2H5, AND C2H4 - THE RATES OF DISPROPORTIONATION AND RECOMBINATION OF ETHYL RADICALS

Using the very low pressure reactor (VLPR) with recent improvements, we have been able to measure the following rate constants over the range 203-343 K: Cl + C2H6 -->1 HCl + C2H5, k1 = (8.20 +/- 0.12) x 10(-11) exp[(-170 +/- 20)/RT]; Cl + C2H5 -->2 HCl + C2H4, k2 = (1.20 +/- 0.08) x 10(-11); 2C2H5 -->3 C2H4 + C2H6, k3 = (2.00 +/- 0.06) x 10(-12); Cl + C2H4 -->4 HCl + C2H3, k4 = (1.1 5 +/- 0.13) x 10(-10) exp[-3200 +/- 140)/RT]. All rate constants are in units of cm3/molecule. and energies are in cal/mol. Reactions 2 and 3 have no observable temperature dependence over the range measured. Combining k3 with the experimentally measured value of the ratio of recombination to disproportionation of 0.14 +/- 0.01, also independent of temperature, yields for the rate of recombination of ethyl radicals: k(r) = (1.42 +/- 0.11) x 10(-11). Combining the values of k4 with the previously measured values of the equilibrium constant K4 and third law corrected values of DELTA-S4 gives for the back reaction: k-4 = (8.7 +/- 0.3) x 10(-13) exp[(-200 +/- 200)/RT] and DELTA-H-degrees-298(C2H3-H) = 106.0 +/- 0.3 kcal/mol. The absence of any measurable butane shows that the recombination rate is at least a factor of 400 below its high-pressure value at 3 mTorr. Mass spectral sensitivity data for ethyl radical between 29 and 25 amu is also reported for 20 and 40 eV.